1. Field of the Invention
This invention relates to a method and system for co-producing fuels, chemicals, and electric power. In one aspect, this invention relates to the use of mechanical expanders and gas turbines for co-producing fuels, chemicals, and electric power. In one aspect, this invention relates to the use of gas turbines for the production of synthetic gas, also referred to as syngas, and electric power, and the subsequent conversion of the syngas to various liquid fuels and/or chemicals.
2. Description of Related Art
There are a variety of known processes for the generation of synthesis gas. U.S. Pat. No. 6,306,917 B1 to Bohn et al. teaches a method and apparatus for producing power, liquid hydrocarbons and carbon dioxide from heavy feedstocks using a partial oxidation reactor to produce synthesis gas, a Fischer-Tropsch (FT) reactor to convert the synthesis gas to hydrocarbon products and tail gases containing hydrogen and carbon dioxide, and a combined cycle plant to produce power from steam generated by recovering heat from the reactors and from combustible tail gases. U.S. Pat. No. 6,596,780 B2 to Jahnke et al. teaches a method for generating syngas comprising H2 and CO2 by gasification of hydrocarbonaceous fuels, such as coal, oil or gas, scrubbing the syngas free of particles, and saturating with water. The syngas is then treated in an acid gas removal unit as desired to remove any impurities in the syngas after which it is routed to a hydrocarbon synthesis reactor in which the H2 and CO2 in the syngas are converted to synthetic hydrocarbons. The unreacted tailgas exiting the reactor is sent to a gas turbine as fuel.
Gas turbines are one of the major sources for power generation in use today. However, the best efficiency achieved to date using simple cycle gas turbines is only about 38%. One significant drawback of gas turbines is that a significant portion of fuel energy input to the gas turbines, approximately 62-75%, is lost in the turbine exhaust. This exhaust energy is in the form of thermal energy only, which makes it difficult to use for effective power generation. Staged reheat gas turbines have the capability to improve both efficiency and NOx emissions. In some gas turbines, fuel staging has been employed. Fuel staging improves system efficiency but has limited application due to combustion instability problems, particularly in the first stage, high NOx emissions, and a large portion of thermal energy, about 55-65%, in the turbine exhaust. U.S. Pat. No. 7,421,835 B2 to Rabovitser et al. teaches a two-stage power generation system having a compressed air source with two compressed air outlets, one of which provides compressed air to the first stage of power generation and the other of which provides compressed air to the second stage of power generation. All of the fuel for the two-stage power generation system is introduced into the first stage. Exhaust gases from the first stage are introduced into a fuel inlet of the second stage of power generation. The first stage preferably includes a gas turbine operated in partial oxidation mode. The exhaust gases from the partial oxidation gas turbine contain thermal and chemical energy, both of which are used in the second stage. In accordance with one embodiment, the exhaust gases are split into two streams, one of which is employed for power generation and the other of which is used for hydrogen production.
For conventional syngas generation processes such as steam methane reforming (STR), shown in FIG. 7, and autothermal reforming (ATR), shown in FIG. 8, involving the reactions of natural gas with oxygen and steam that uses a catalytic reactor or a non-catalytic partial oxidation (POx) reactor, the outlet temperature of the syngas is limited, typically to about 1600-1700° F. at pressures of about 20-40 atm, due to limitations of the metallurgy for the equipment such as a waste heat boiler for production of steam used in the syngas cooling step, and due to the catalysts used for reforming reactions.